'Latek' is a dimunitive form of the noun "lat" and it's pretty normal when asking children how old they are.
Other than that, it can be used ironically in reference to almost everyone, as Marco has rightly pointed out.

One example that comes to mind is a teacher talking to his or her students, angry with them for not having carried out some task:Nie macie już 5 latek żebyby zachowywać się tak nieodpowiedzialnie! - You're not five anymore, to behave in such an irresponsible way!